1. Field
The present invention relates to an optical line testing device such as an Optical Time Domain Reflectometer (OTDR), and more particularly, to an optical line testing device using a wavelength tunable laser.
2. Description of the Related Art
As communication volume increases, copper-based communication cables are replaced by optical fiber-based optical lines. The optical lines have been installed only in a section connecting a telephone office and a telephone office, but due to the increase in multimedia services such as Video On Demand (VOD), the optical lines are now being installed in homes or each room of the homes like a Fiber To The Home (FTTH). Therefore, as a service provider, management of numerous optical lines and detection of fault points have become very important in management of a communication network.
One of devices for managing the optical lines is an optical line testing device, for example, an Optical Time Domain Reflectometer (OTDR). As shown in FIG. 1, the OTDR generates an optical pulse 2 whose power is high and whose width is short in a laser 1, and enters the optical pulse 2 into the optical line 3 to be tested to start a test. If there is a fine-cut surface 4 somewhere in the optical line 3, the optical pulse 2 here makes a reflection pulse opposite to the traveling direction, receives the reflection pulse again, and generally displays the result like FIG. 2. Since an operating principle of the OTDR corresponds to a known technology, a detailed description thereof will not be given herein.
(References: Korean Patent Publications No,. 2004-23305 and No. 1997-28648).
However, a classical OTDR using an optical pulse is a useful tool for managing quality of an optical line in most cases, but has the following disadvantages.
First, it is difficult to increase a dynamic range. The dynamic range refers to a distance the OTDR can measure. To increase the range, it is necessary to increase magnitude of the optical pulse. However, if the magnitude of the optical pulse is increased beyond a threshold value, a nonlinear effect due to an interaction between the optical line and the optical pulse is strongly generated, and a shape of the optical pulse is distorted to cause a measurement error.
At present, to avoid such an error, a length (width) of the optical pulse is increased instead of increasing the magnitude of the optical pulse. This increases the dynamic range. However, as the length (width) of the optical pulse increases, a resolution of the OTDR deteriorates as shown in FIG. 3. The resolution is improved as the length of the optical pulse is shorter. The resolution is represented by parameters such as an event dead zone and an attenuation dead zone and all of the parameters are connected to each other, so that if one property is improved, the other property is damaged.
Furthermore, an Erbium Doped Fiber Amplifier (EDFA) may be used as an alternative method for increasing the dynamic range. However, since a conventional method of the OTDR uses an optical pulse having extreme variations in optical power over time, it is inappropriate to use the EDFA for amplifying the optical pulse.
As described above, according to the prior art, there is a limit to further improve the dynamic range and the resolution, and therefore, a technique capable of solving the problem is required.